In the industry monochloroacetic acid is prepared by reacting acetic acid with chlorine. Such a process for the preparation of monochloroacetic acid is commonly known and generally makes use of a reactor in which a mixture of liquid acetic acid (HAc) is reacted with chlorine gas under anhydrous conditions. A large number of compounds can be used to create these anhydrous conditions. If acetic anhydride is used, this will immediately be converted with hydrochloric acid into acetyl chloride, which is the catalyst for this process. The process generally is conducted at a pressure of from 1 to 6 barA and a temperature of from 80 to 180° C. In the reactor, monochloroacetic acid (MCA) and gaseous HCl are formed together with by-products of which dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are examples.
After the MCA-containing reaction product mixture has passed the reactor(s) and the catalyst recovery section, DCA is present in a significant amount, typically about 3-10%. The MCA/DCA-containing product mixture is subsequently led to a unit to reduce the amount of DCA in the MCA. This can be done by a physical separation, such as melt crystallization, or by a chemical conversion, such as a reduction where DCA is reduced with hydrogen in the presence of a hydrogenation catalyst, e.g. a metal-based catalyst. This catalyst not only reduces DCA, but also reduces MCA to some extent, which is of course undesirable. Moreover, such a reduction unit and its operation are expensive, and this adds to the production costs of the MCA end product.
The low-boiling components are then removed from the MCA by conventional vacuum distillation.
A process to prepare MCA by an electrochemical process has been disclosed by A. Youtz et al., “Depolarization of the Chlorine Electrode by Organic Compounds” in J. Am. Chem. Soc., 1924, 46, 549. The process encompasses the reaction of 70% acetic acid in an aqueous solution with hydrochloric acid to give chloroacetic acid.
An electrochemical process to prepare a halogenated carboxylic acid (derivative) in an aqueous environment has the disadvantage that only a low amount of the desired (mono)halogenated carboxylic acid is formed. Besides, in an aqueous chloride environment the main product of electrolysis is often chlorine gas, which is undesirable, as it represents an additional waste stream and besides the combination of chlorine gas and hydrogen gas may make the reaction's off-gas mixture explosive.